The earliest implantable medical devices, e.g., implantable cardiac pacemakers and other body tissue stimulating devices, were formed of an implantable pulse generator (IPG) and a set of electrical leads attached between the IPG and heart or body tissue to be paced or stimulated. Typically, the IPG electrical circuit was powered either by Hg--Zn batteries or by induction of energy transmitted transcutaneously from a skin surface RF power generator and supplied electrical pacing or stimulating pulses to the leads. The IPG batteries and circuits were encapsulated within an epoxy compound partly for ease of manufacture and to allow hydrogen emitted by the Hg--Zn batteries to escape. Electrical connector pins and rings, if present, were initially permanently attached to the circuits. Other early implantable medical devices, e.g. implantable monitors and cochlear implants or the like were also formed in somewhat the same manner.
Such early implantable cardiac pacemakers suffered very short useful lives due to moisture ingress through the epoxy and causing electrical dendritic growth across, and shorting of, adjacent points of the circuit, battery terminals, or discrete transistor terminals. In addition, pacing leads frequently failed due to conductor stress fractures, and batteries depleted prematurely for a variety of reasons.
In the 1960's, IPG connector assemblies were formed integrally with other IPG circuit components and embedded in an epoxy housing to enable attachment of a chosen lead to the IPG circuit for initial implant or defective lead replacement purposes. These integrally formed connector assemblies typically comprised at least one metal, electrical connector block encapsulated therein that were aligned in relation to an elongated lead end receptacle for receiving the proximal lead end. Each connector block was formed to have a bore to receive the lead connector pin or ring, depending on the type of lead intended to be used, and a threaded cross bore receiving a trapped set screw. The connector block(s) was electrically directly attached to the IPG circuit. A silicone rubber suture boot was placed in the mold so that it's bore would be aligned with the elongated receptacle. The entire IPG including these connector assembly components, was then encapsulated in epoxy.
In use, the proximal lead connector end(s) was inserted into the appropriate lead connector receptacle until the lead connector pin or ring was received in the bore of the connector block(s). Then, the set screw was tightened by a hex wrench to maintain the firm electrical and mechanical attachment, and the opening through the molded epoxy housing to access the set screw was sealed. Sutures were tied around the suture boot(s) to seal it against the lead body.
Since about the mid-1970's, hermetically sealed, lithium batteries and miniaturized, digital and analog, integrated circuits (ICs) have been used in implantable medical devices, particularly for implantable cardiac pacemaker and nerve stimulation IPGs. The ICs, batteries, and other components are enclosed in hermetically sealed metallic enclosures or "cans" separated from the connector assembly components. Electrical connection with the connector block(s) and/or other components of the connector assembly is obtained by use of electrical feedthroughs supporting feedthrough pins that extend through the hermetically sealed can.
In certain instances, the lead connector assembly components external to the hermetically sealed enclosure are still to this date attached to an attachment surface thereof using an in situ molding process to seal the connector assembly components and form the receptacle for a lead or catheter proximal end, etc. Very simply, in the formation of a lead connector assembly for a cardiac pacemaker IPG, for example, the connector block(s) and feedthrough pin(s) are welded together and laid out in a mold with respect to any other associated components and mold plugs. An encapsulating compound is injected into the mold to form the connector header assembly molded to the IPG attachment surface as described, for example, in U.S. Pat. No. 4,041,956. This approach is time consuming and not terribly precise. If the resulting connector header assembly fails to meet dimensional tolerances or other quality requirements, it is difficult to rework the IPG.
In 1979, the MEDTRONIC.RTM. SPECTRAX.RTM. cardiac pacemaker IPGs were introduced having the digital and analog or hybrid IC's and lithium batteries forming the pacing circuit enclosed within a hermetically sealed titanium enclosure having feedthroughs extending through an enclosure attachment surface thereof. Such an assembly of these components is disclosed in commonly assigned U.S. Pat. Nos. 4,142,532 and 4,182,345, incorporated herein by reference.
The lead connector assembly, in this case and as used in IPG models to the present time by Medtronic, Inc., is manufactured as a separate pre-formed connector header module that encloses connector components and is attached to an enclosure attachment surface of the hermetically sealed enclosure and to the feedthrough pins. The connector header module is molded of a thermoplastic elastomer, e.g., a medical grade polyurethane, with an outer module surface and a number of receptacles and channels within it that in some instances are accessible through windows extending to the module surface. The connector header module receives the electrical connector block(s) in connector block receptacle(s) such that the connector block bore(s) is aligned with elongated lead connector receptacle(s) for receiving the proximal lead connector end assembly(s). In the typical design, each such connector block is formed with a threaded cross bore receiving a trapped set screw as described above. Each set screw of each connector block in a connector block receptacle is also aligned with a septum receptacle for receiving a silicone rubber set screw septum. The pre-formed connector header module is formed with pin channel(s) for directing the feedthrough pin(s) into contact with the respective connector block(s) and with windows) to allow the connector block(s) and septum(s) to be inserted into their respective receptacle(s). In each case, the connector block receptacle window or a further window to the module surface is provided for allowing the feedthrough pin end to be welded to the connector block. The windows(s) and pin channel(s) are typically back filled with a medical grade silicone adhesive after the welding step and attachment of the connector header module to the hermetically sealed enclosure.
To attain and maintain these characteristics, the receptacle for the connector block and the connector block itself are dimensioned within tight tolerances to precisely align the connector block bore with the lead connector receptacle. In one approach, the connector block receptacle opening dimensions are reduced and the opening edge shaped so that the connector block stretches the opening edge as it is inserted to be seated within the connector block receptacle. In certain other cardiac pacemaker IPGs, each connector block is inserted into a connector block receptacle and ultrasonic energy is applied to the edge of the connector block window to melt it over and tamp it against the exposed surface of the connector block. This ultrasonic tamping technique of dissimilar material parts is similar to that shown in the article entitled "Ultrasonic joining of moulded parts and semi-finished parts of thermo-plastic polymers in mass production--Forming wit ultrasound. Staking, swaging and tamping (Guideline DVS 2216, Part 3, 1992)", Welding in the World, Le Soudage Dans Le Monde, Vol. 31, No. 3, pp. 205-207 (1993).
As a general rule, the connector header module formed as described above, has to satisfy very tight tolerances and remain dimensionally stable over a long time period of implantation within the hostile environment of the human body. Any substantial initial or time-induced misalignment of the lead connector receptacle bore(s) extending through the molded module housing and the connector block bore(s) can make initial attachment or removal and replacement of a lead connector end impossible or unreliable.
During the attachment of the connector header module to the hermetically sealed enclosure, medical grade adhesive may be employed to adhere the module attachment surface with the enclosure attachment surface. During the curing of the adhesive, it is necessary to ensure that the attachment surfaces are not disturbed. It has been proposed to employ mechanical attachment mechanisms as a substitute for or in addition to the use of the medical grade adhesive between the attachment surfaces to provide stability and strength. Such mechanical attachment mechanisms that have been proposed for use or actually used either alone or with adhesive take a variety of forms, e.g., the forms shown in commonly assigned U.S. Pat. Nos. 4,142,532 and 4,182,345, both incorporated herein by reference in their respective entireties. While these approaches have merit, they require use of additional, precision piece parts and assembly steps that can add to the cost and time spent in assembling the connector header module with the hermetically sealed enclosure.
In a further current approach employing both adhesive and temporary mechanical fixation, tab channels are formed in the connector header module to receive upstanding attachment tabs that are welded to the enclosure attachment surface to extend outward therefrom. The upstanding attachment tabs are formed with hooks at their ends that snap into engagement with undercuts that are intentionally formed in the tab channels. Adhesive is applied to the mating attachment surfaces and into the tab channels. This mechanical attachment provides a temporary fixation with relatively low resistance to dislodgement for several hours until adhesive applied to the mating attachment surfaces sets up.
Finally, it should be noted that it has been recently proposed to form the connector header module as part of a shroud surrounding and adhering to the rim of the hermetically sealed enclosure in order to simplify the assembly by reducing the number of parts, assembly steps and required tolerances. Such a configuration is shown in commonly assigned U.S. Pat. Nos. 5,535,097, 5,522,861, 5,456,698 and 5,431,695, all incorporated herein by reference in their respective entireties. In this configuration, the shroud is preferably formed of a flexible silicon rubber, and pacing leads may be attached and replaced in the normal manner for an implantable pacing system. However, the use of silicone rubber has its own disadvantages related to dimensional instability and lack of rigidity, general design aesthetics, and potential discoloration of the silicone rubber during storage and sterilization which contribute to lack of market acceptance.
The adhesion force or "peel and pull strength" of the connector header module with the hermetically sealed enclosure at the mating attachment surfaces is important both during the adhesive curing time and in later chronic implantation of the implantable medical device. The pull strength is the separation force, specified in pounds, applied perpendicularly to the plane of the header module and enclosure attachment surfaces sufficient to pull the header module away from the hermetically sealed enclosure. The peel strength is the separation force, specified in pounds, applied laterally against a major side of the header module sufficient to break the attachment laterally. The curing or drying of adhesive between the mating attachment surfaces to establish a specified peel and pull strength takes time, and this delay in manufacturing is undesirable. Moreover, the peel and pull strength that can be achieved is limited by the areas of contact of the mating attachment surfaces. Consequently, a need remains for an improved mechanical attachment mechanism and method that avoids undue delay and provides enhanced peel and pull strength upon completing the attachment. As will become apparent from the following, the present invention satisfies that need.
Certain implantable medical devices other than IPGs also require the attachment of a pre-formed header module with a hermetically sealed enclosure. Such pre-formed header modules do not include lead connector components and connector end receptacles. Similar considerations of simplified manufacture, reduced cost and enhanced peel and pull strength also apply to the assembly of such pre-formed header modules and hermetically sealed enclosures.